1. Field of the Invention
The present invention relates to a thin film transistor and a fabrication method of the same and, particularly, to a thin film transistor for use in an active matrix type liquid crystal display panel and a fabrication method of the same.
2. Description of the Prior Art
Active matrix type liquid crystal display panels have been in great demand recently. To meet the demand, it becomes necessary, in order to improve the producibility thereof, to reduce the number of patterning steps in fabricating thin film transistors. Particularly, it is of urgent necessity to reduce the number of patterning steps without degrading a display quality of a resultant liquid crystal display panel.
In a conventional usual fabrication method of a thin film transistor (TFT), the number of patterning steps of which is reduced, at least 5 (five) patterning steps are required by using at least five kinds of photo resist patterns to be described below, which are used in fabricating a reverse-staggered type TFT. The reverse-staggered type TFT is also referred to as xe2x80x9cbottom gate type TFTxe2x80x9d.
The first one of the five patterning steps is to pattern a gate wiring after an electrically conductive film for gate wiring is formed on a glass substrate.
The second patterning step is to pattern a gate insulating film and a semiconductor layer formed on the gate wiring in the order such that a portion of the semiconductor and an ohmic contact layer, which constitutes a transistor, are selectively provided.
The third patterning step is to pattern a source and drain electrode film to provide source and drain wirings. In this step, a portion of the ohmic semiconductor layer exposed between the source electrode and the drain electrode is etched away.
The fourth patterning step is to form a contact-hole connecting a pixel electrode to either the drain electrode or the source electrode by patterning a nitride passivation film formed on a whole surface of the wafer.
The fifth patterning step is to pattern a transparent pixel electrode film formed on the whole surface of the wafer to provide a transparent pixel electrode.
In this specification, one of the source and drain electrodes, both of which are A. C. driven, connected to the pixel electrode will be referred to as the source electrode and the other will be referred to as the drain electrode.
As mentioned, in the conventional fabrication method of the thin film transistor, at least five patterning steps are required. Furthermore, when a back channel of the TFT is in a floating state and the TFT is operating for a long time, leakage current of the back channel is increased, causing display on a screen of the liquid crystal display panel to be uneven.
An object of the present invention is to provide a thin film transistor having a structure with which the number of patterning steps with using photo resists can be reduced and a fabrication method of the same thin film transistor.
Another object of the present invention is to provide a thin film transistor capable of reducing leakage current of a back channel when it is operated continuously and a fabrication method of the same thin film transistor.
According to the present invention, a thin film transistor having a back channel electrode is featured by that a voltage of a front channel positioned on the side of a gate wiring of the thin film transistor is made equal to a voltage of the back channel positioned on the side of a back channel electrode by short-circuiting the back channel electrode to a gate electrode through a contact-hole provided in a portion of a semiconductor layer constituting the thin film transistor.
The back channel electrode is featured by that it is formed of a material, which is a material of a pixel electrode such as transparent electrode connected to one of a source electrode and a drain electrode of the thin film transistor.
The contact-hole is preferably formed in a location remote from an active region of the thin film transistor by at least 5 microns.
Furthermore, the present invention is featured by that a passivation film patterned to have a width equal to that of the back channel electrode and the semiconductor layer are provided between the back channel and a gate insulating film.
Moreover, the present invention is featured by that the semiconductor layer patterned to have a width equal to that of the source and drain electrodes of the thin film transistor is provided between the source and drain electrodes and the gate insulating film.
The semiconductor layer is featured by that it has an ohmic contact layer on the side thereof, which is in contact with the source and drain electrodes.
In addition, according to the present invention, a thin film transistor having a semiconductor layer formed on a gate insulating film which is formed on a gate electrode wiring, source and drain wirings formed on the semiconductor layer and a back channel electrode formed on a passivation film which is formed on the source and drain wirings is featured by that a pixel electrode connected to one of the source electrode and the drain electrode is formed of the same material as that of the back channel electrode simultaneously with formation of the back channel electrode, the passivation film patterned to have a pattern identical to that of the back channel electrode and the semiconductor layer are provided between the back channel and a gate insulating film, the back channel electrode and a gate electrode are connected each other through a contact hole penetrating the passivation film, the semiconductor layer and the gate insulating film and the semiconductor layer patterned to have a pattern identical to that of a source and drain wiring layer is provided between the source and drain wiring layer and the gate insulating film.
The thin film transistor is featured by that a side face of one of the source electrode and the drain electrode, which is connected to a pixel electrode, is in contact with the pixel electrode and, further, the whole side face of the one electrode is in contact with the pixel electrode.
According to the present invention, a fabrication method of the above mentioned thin film transistor, which comprises the steps of forming a gate electrode wiring pattern on a substrate, forming a semiconductor layer and source and drain electrodes on a gate insulating film, forming a pixel electrode connected to one of the source electrode and the drain electrode and forming a back channel electrode on a passivation film formed on an active region of the thin film transistor, is featured by comprising the steps of patterning the source and drain electrodes without patterning the semiconductor layer, forming the passivation film after the patterning step, patterning a gate contact hole for connecting the back channel electrode to the gate electrode and an opening portion for the pixel electrode such that the gate contact hole and the opening portion penetrate the passivation film, the semiconductor layer and the gate insulating film, forming an electrically conductive film for the pixel electrode such that the conductive film commonly covers the gate contact hole and the opening portion and patterning the conductive film such that the pixel electrode and the back channel electrode are left as they are, wherein the passivation film and the semiconductor layer, which are not removed, are patterned simultaneously with using the pixel electrode, the back channel electrode and the source and drain electrodes, which are not removed, as a mask.
In this fabrication method, the opening portion is provided by removing a portion of the one of the source electrode and the drain electrode, to which the pixel electrode is connected.
The contact hole is formed in a location remote from the active region of the thin film transistor.
Furthermore, in this fabrication method, a one side of the opening portion is formed such that one side of one of the source electrode and the drain electrode is exposed through the opening portion and is electrically connected to the pixel electrode.
In addition, the semiconductor layer has an ohmic contact layer on the side of the source and drain electrodes and the ohmic contact layer on the active region is patterned simultaneously with the patterning step of the source and drain electrodes.
Moreover, according to the present invention, the fabrication method of the thin film transistor is featured by comprising the first patterning step of forming a first wiring pattern on a substrate, the step of forming a lamination of a first insulating film, a semiconductor layer and a second wiring film on the substrate and the first wiring pattern, the second patterning step of patterning the second wiring film to form a predetermined wiring pattern, the step of forming a second insulating film commonly on the lamination and the second wiring pattern, the third patterning step of simultaneously forming a gate contact hole for exposing a portion of the first wiring pattern through the second insulating film and an opening portion for a pixel electrode for exposing a portion of the substrate through the second insulating film, the step of forming a transparent electrode and the fourth patterning step of forming a transparent pixel electrode by patterning the transparent electrode and, simultaneously therewith, defining a semiconductor region by selectively etching the semiconductor layer by using a resist mask on the transparent electrode and the second wiring pattern as an etching resist mask.